The choices of English words includes "sons" but not "sons'". The apostrophe is missing. Maybe duolingo can't have a choice word end with an apostrophe, as I've seen the same problem with other questions.
In English this should be either: our son's dog (if we have only one son) or: our sons' dog (if we have sevaral sons who share a dog.
Here it is plural - "synů" - so the English sentence needs "sons.'" (apostrophe at the end...)